Agricultural equipment, such as a tractor or a self-propelled combine-harvester, includes a prime mover which generates power to perform work. In the case of a tractor, the prime mover is a gas powered engine or a diesel engine that generates power from a supply of fuel. The engine drives a transmission which moves wheels or treads to propel the tractor, or other work vehicles, across ground or other surfaces. In addition to providing power to wheels through a transmission, tractors often include a power takeoff (PTO) which includes a shaft coupled to the transmission and which is driven by the engine or a hydraulic motor.
Both gas powered and diesel powered vehicles includes a fluid management system to manage the movement of a hydraulic fluid in the transmission. In different embodiments, the fluid management system manages the movement of fluid to other vehicle systems and components including a differential, a steering system, brakes, the PTO, and various fluid reservoirs and coolers. Other systems and components which use the managed fluid include a suspension, a hitch, a cab suspension, and selective control valves.
Fluid management systems typically include a positive displacement pump which transfers the fluid between different portions of the fluid management system, for instance between a differential and a hydraulic reservoir. In one known configuration, the hydraulic system and the drivetrain share the hydraulic fluid and the drivetrain system circulates fluid through a fluid cooler. In one embodiment, a transfer pump, typically a gear pump, moves the common fluid from the differential case to the hydraulic reservoir through a hydraulic filter.
Energy losses occur throughout the drivetrain when the vehicle is moving or standing still. This energy loss, also known as a parasitic loss, is a result of many factors including the movement of oil in the system which exceeds the flow requirements. The excess flow is returned to a reservoir or a transmission sump, and is consequently not converted to perform a function but instead results in lost energy.
Transfer pumps under different operating conditions can be a source of these parasitic losses, even when the vehicle's engine is idling. Attempts to reduce the parasitic losses of positive displacement pumps have been provided by the use of additional devices such as expensive clutches, external unloading valves, large unloading valves, piston pumps, and variable displacement mechanisms for vane pumps. These solutions, however, still develop pressure losses. Consequently, what is needed therefore is a transfer pump that further reduces parasitic losses without the use of costly additional devices or the additional complexity associated with such devices.